Immune Modulation and Immune-Mediated Pathogenesis of Emerging Tickborne Banyangviruses

Development and validation of the PLNA score to predict cytokine storm in acute-phase SFTS patients: A single-center cohort study

BACKGROUND

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease known for its high mortality rate and its correlation with Cytokine Storms (CS). Timely detection of CS is crucial for improving the prognosis of the disease. The objective of this investigation was to develop a model for identifying cytokine storms in the acute phase of SFTS.

METHODS

A total of 245 patients diagnosed with SFTS were included in this study between January 2020 and July 2022. Among them, 184 patients were part of the training set, while 61 patients were part of the validation set. Variables identified by LASSO were subsequently included in a multivariate logistic regression analysis to determine independent predictors. Subsequently, a nomogram was then developed to predict the likelihood of CS in SFTS patients. The predictive efficacy and clinical applicability of the nomogram model were further assessed through ROC analysis and the DCA curve.

RESULTS

Following LASSO analysis, a total of 11 indicators were included in multivariate logistic regression analysis. The findings indicated that PLT (OR 0.865, P < 0.001), LDH (OR 1.002, P < 0.001), Na+ (OR 1.155, P = 0.005), and ALT (OR 1.019, P < 0.001) serve as independently predictors of CS in the acute phase of SFTS. Furthermore, a nomogram named the PLNA was constructed by integrating these four factors. The PLNA model exhibited favorable predictive accuracy with an AUC of 0.958. Moreover, the PLNA model exhibited excellent clinical applicability in both the training and validation sets, as evidenced by the DCA curve.

CONCLUSIONS

The PLNA model, constructed using clinical indicators, can predict the probability of cytokine storm in the acute phase of SFTS patients.

MCP-3 as a prognostic biomarker for severe fever with thrombocytopenia syndrome: a longitudinal cytokine profile study

Introduction

Severe fever with thrombocytopenia syndrome (SFTS) is characterized by a high mortality rate and is associated with immune dysregulation. Cytokine storms may play an important role in adverse disease regression, this study aimed to assess the validity of MCP-3 in predicting adverse outcomes in SFTS patients and to investigate the longitudinal cytokine profile in SFTS patients.

Methods

The prospective study was conducted at Yantai Qishan Hospital from May to November 2022. We collected clinical data and serial blood samples during hospitalization, patients with SFTS were divided into survival and non-survival groups based on the clinical prognosis.

Results

The levels of serum 48 cytokines were measured using Luminex assays. Compared to healthy controls, SFTS patients exhibited higher levels of most cytokines. The non-survival group had significantly higher levels of 32 cytokines compared to the survival group. Among these cytokines, MCP-3 was ranked as the most significant variable by the random forest (RF) model in predicting the poor prognosis of SFTS patients. Additionally, we validated the predictive effects of MCP-3 through receiver operating characteristic (ROC) curve analysis with an AUC of 0.882 (95% CI, 0.787-0.978, P <0.001), and the clinical applicability of MCP-3 was assessed favorably based on decision curve analysis (DCA). The Spearman correlation analysis indicated that the level of MCP-3 was positively correlated with ALT, AST, LDH, α-HBDH, APTT, D-dimer, and viral load (P<0.01).

Discussion

For the first time, our study identified and validated that MCP-3 could serve as a meaningful biomarker for predicting the fatal outcome of SFTS patients. The longitudinal cytokine profile analyzed that abnormally increased cytokines were associated with the poor prognosis of SFTS patients. Our study provides new insights into exploring the pathogenesis of cytokines with organ damage and leading to adverse effects.

Computer-aided designing of a novel multi‑epitope DNA vaccine against severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne viral disease caused by the SFTS virus (Dabie bandavirus), which has become a substantial risk to public health. No specific treatment is available now, that calls for an effective vaccine. Given this, we aimed to develop a multi-epitope DNA vaccine through the help of bioinformatics. The final DNA vaccine was inserted into a special plasmid vector pVAX1, consisting of CD8+ T cell epitopes, CD4+ T cell epitopes and B cell epitopes (six epitopes each) screened from four genome-encoded proteins--nuclear protein (NP), glycoprotein (GP), RNA-dependent RNA polymerase (RdRp), as well as nonstructural protein (NSs). To ascertain if the predicted structure would be stable and successful in preventing infection, an immunological simulation was run on it. In conclusion, we designed a multi-epitope DNA vaccine that is expected to be effective against Dabie bandavirus, but in vivo trials are needed to verify this claim.

Comparative Analysis of Bisexual and Parthenogenetic Populations in Haemaphysalis Longicornis

Haemaphysalis longicornis, a three-host tick with a wide host range, is widely distributed in different countries and regions. It stands out among ticks due to its unique feature of having both parthenogenetic and bisexual populations. Despite their morphological resemblance, the characteristics of the parthenogenetic population have been overlooked. In this comprehensive study, we systematically compared the similarities and differences between these two populations. Our investigation revealed that the parthenogenetic H. longicornis, widely distributed in China, was found in ten provinces, surpassing the previously reported distribution. Notably, individuals from the parthenogenetic population exhibited a prolonged blood-feeding duration during the larval and nymph stages compared to their bisexual counterparts. Additionally, the life cycle of the parthenogenetic population was observed to be longer. A flow cytometry analysis indicated a DNA content ratio of approximately 2:3 between the bisexual and parthenogenetic populations. A phylogenetic analysis using whole mitochondrial genome sequences resulted in the separation of the phylogenetic tree into two distinct branches. A molecular analysis unveiled a consistent single T-base deletion at nucleotide 8497 in the parthenogenetic population compared to the bisexual population. Both populations displayed high viral infection capability and significant resistance to ivermectin. Intriguingly, despite these differences, the parthenogenetic population exhibited a similar life cycle to the bisexual population, retaining the ability to transmit pathogens such as Severe fever with thrombocytopenia syndrome virus (SFTSV) and Heartland Virus (HRTV). These findings contribute to a deeper understanding of the distinct characteristics and similarities between different populations of H. longicornis, laying the foundation for future research in this field.

Heartland Virus Disease-An Underreported Emerging Infection

First recognized 15 years ago, Heartland virus disease (Heartland) is a tickborne infection contracted from the transmission of Heartland virus (HRTV) through tick bites from the lone star tick (Amblyomma americanum) and potentially other tick species. Heartland symptoms include a fever <100.4 °F, lethargy, fatigue, headaches, myalgia, a loss of appetite, nausea, diarrhea, weight loss, arthralgia, leukopenia and thrombocytopenia. We reviewed the existing peer-reviewed literature for HRTV and Heartland to more completely characterize this rarely reported, recently discovered illness. The absence of ongoing serosurveys and targeted clinical and tickborne virus investigations specific to HRTV presence and Heartland likely contributes to infection underestimation. While HRTV transmission occurs in southern and midwestern states, the true range of this infection is likely larger than now understood. The disease's proliferation benefits from an expanded tick range due to rising climate temperatures favoring habitat expansion. We recommend HRTV disease be considered in the differential diagnosis for patients with a reported exposure to ticks in areas where HRTV has been previously identified. HRTV testing should be considered early for those matching the Heartland disease profile and nonresponsive to initial broad-spectrum antimicrobial treatment. Despite aggressive supportive therapy, patients deteriorating to sepsis early in the course of the disease have a very grim prognosis.

Changes in Cytokine Levels in Patients with Severe Fever with Thrombocytopenia Syndrome Virus

Purpose

To characterize the cytokine profile of patients with severe fever with thrombocytopenia syndrome (SFTS) in relation to disease severity.

Patients and Methods

60 laboratory-confirmed SFTS patients and 12 healthy individuals from multi-centers in Shandong Province of China were included, and all patients were divided into fatal patients (9) and recovered patients (51) due to their final outcomes. Multiplex-microbead immunoassays were conducted to estimate levels of 27 cytokines in the sera of patients and controls.

Results

The results showed that levels of IL-2, IL-4, IL-6, IL-7, IL-8, IL-15, IL-1RA, G-CSF, GM-CSF, IFN-γ, TNF-α, basic FGF, PDGF-BB, RANTES, IP-10, MIP-1α, MIP-1β, MCP-1, and Eotaxin differed significantly among the SFTS fatal patients, recovered patients, and the healthy controls (all p<0.05). Compared to the healthy controls, the fatal patients and recovered patients had reduced levels of IL-2, IL-4, IL-7, PDGF-BB, RANTES, and Eotaxin, while the levels of PDGF-BB and RANTES were significantly lower in fatal patients compared to recovered patients. The increasing levels of IL-6, IL-8, IL-15, IL-1RA, G-CSF, GM-CSF, IFN-γ, TNF-α, basic FGF, IP-10, MIP-1α, MIP-1β, and MCP-1 were observed in fatal patients (all p<0.05), and the levels of IL-6, IP-10, MIP-1α, and MCP-1 were significantly higher than other two groups. The Spearman correlation analysis indicated a positive correlation between platelet count and PDGF-BB levels (p<0.05), while the white blood cell count had a negative correlation with MIP-1 level (p<0.05).

Conclusion

The research exhibited that the SFTS virus (SFTSV) caused an atypical manifestation of cytokines. The levels of IL-6, IP-10, MIP-1α, and MCP-1 had been observed a positive association with the severity of the illness.

Regulation of the WNT-CTNNB1 signaling pathway by severe fever with thrombocytopenia syndrome virus in a cap-snatching manner

The segmented negative-strand RNA viruses (sNSVs) include highly pathogenic human and animal viruses such as Lassa virus (LASV), severe fever with thrombocytopenia syndrome virus (SFTSV), and influenza A virus (IAV). One of the conserved mechanisms at the stage of genome transcription of sNSVs is the cap-snatching process, providing druggable targets for the development of antivirals. SFTSV is an emerging tick-borne sNSV that causes severe hemorrhagic fever with a high fatality rate of 12%-50%. Here, we determined the correlation between death outcome and downregulation of the WNT-CTNNB1 signaling pathway through transcriptomic analysis of blood samples collected from SFTS patients. We further demonstrated that SFTSV affected this pathway by downregulating the mRNA levels of a series of pathway-related genes, including CTNNB1. Loss-of-function mutations or inhibitors targeting SFTSV cap-snatching activity effectively alleviated the inhibition of the WNT-CTNNB1 signaling pathway. Exogenous activation of the WNT-CTNNB1 signaling pathway enhanced SFTSV replication, while inhibition of this pathway reduced SFTSV replication. Treatment with a WNT-CTNNB1 signaling pathway inhibitor attenuated viral replication and decreased fatality in mice. Notably, downregulation of the WNT-CTNNB1 signaling pathway was also observed for other sNSVs, including LASV and IAV. These results suggested that RNAs related to the WNT-CTNNB1 signaling pathway might be utilized as a primer "pool" in a cap-snatching manner for viral transcription, which provides effective targets for the development of broad-spectrum antivirals against sNSVs.IMPORTANCEOne of the conserved mechanisms at the stage of genome transcription of segmented negative-strand RNA viruses (sNSVs) is the cap-snatching process, which is vital for sNSVs transcription and provides drugable targets for the development of antivirals. However, the specificity of RNAs snatched by sNSV is still unclear. By transcriptomics analysis of whole blood samples from SFTS patients, we found WNT-CTNNB1 signaling pathway was regulated according to the course of the disease. We then demonstrated that L protein of severe fever with thrombocytopenia syndrome virus (SFTSV) could interact with mRNAs of WNT-CTNNB1 signaling pathway-related gene, thus affecting WNT-CTNNB1 signaling pathway through its cap-snatching activity. Activation of WNT-CTNNB1 signaling pathway enhanced SFTSV replication, while inhibition of this pathway decreased SFTSV replication in vitro and in vivo. These findings suggest that WNT-associated genes may be the substrate for SFTSV "cap-snatching", and indicate a conserved sNSVs replication mechanism involving WNT-CTNNB1 signaling.

Dynamics of neutralizing antibodies against severe fever with thrombocytopenia syndrome virus

OBJECTIVES

Severe fever with thrombocytopenia syndrome (SFTS) virus (SFTSV) is an emerging tick-borne bunyavirus with a high pathogenicity. Little is known about the longitudinal dynamics of the SFTSV-specific neutralizing antibody (NAb) and the related factors in patients with SFTS.

METHODS

A prospective cohort study of patients with laboratory-confirmed SFTS were conducted. Antiglomerulonephritis-immunoglobulin G (anti-Gn-IgG) and NAb titers were examined in serially collected serum samples, and their dynamic features were analyzed.

RESULTS

NAb was initially detected at 15 days after symptom onset in surviving patients with SFTS, with positive rates of 37.21% (16/43), whereas neither anti-Gn-IgG antibody nor NAb was detected in patients with fatal SFTS during their hospitalization. The NAb levels reached the peak at 2 months after symptom onset, and then gradually declined, with a rapid downward trend from 6 months to 4 years and a relatively slow downward trend from 5 to 10 years. There was a positive correlation between NAb and anti-Gn-IgG titers in surviving patients with SFTS (r = 0.699, P <0.001). Patients with a mild illness or low viral load experienced early NAb seroconversion. Six different dynamic patterns of NAb were noted in surviving patients.

CONCLUSION

These data provide useful information regarding the dynamic changes in NAb in patients with SFTS during the acute and convalescent phases and the follow-up period.

Construction and Characterization of Severe Fever with Thrombocytopenia Syndrome Virus with a Fluorescent Reporter for Antiviral Drug Screening

Severe fever with thrombocytopenia syndrome (SFTS) caused by a novel bunyavirus (SFTSV) is an emerging infectious disease with up to 30% case fatality. Currently, there are no specific antiviral drugs or vaccines for SFTS. Here, we constructed a reporter SFTSV in which the virulent factor nonstructural protein (NSs) was replaced by eGFP for drug screening. First, we developed a reverse genetics system based on the SFTSV HBMC5 strain. Then, the reporter virus SFTSV-delNSs-eGFP was constructed, rescued, and characterized in vitro. SFTSV-delNSs-eGFP showed similar growth kinetics with the wild-type virus in Vero cells. We further detected the antiviral efficacy of favipiravir and chloroquine against wild-type and recombinant SFTSV by the quantification of viral RNA, and compared the results with that of fluorescent assay using high-content screening. The results showed that SFTSV-delNSs-eGFP could be used as a reporter virus for antiviral drug screening in vitro. In addition, we analyzed the pathogenesis of SFTSV-delNSs-eGFP in interferon receptor-deficient (IFNAR^-/-) C57BL/6J mice and found that unlike the fatal infection of the wild-type virus, no obvious pathological change or viral replication were observed in SFTSV-delNSs-eGFP-infected mice. Taken together, the green fluorescence and attenuated pathogenicity make SFTSV-delNSs-eGFP a potent tool for the future high-throughput screening of antiviral drugs.

Longitudinal analysis of immunocyte responses and inflammatory cytokine profiles in SFTSV-infected rhesus macaques

Severe fever with thrombocytopenia syndrome virus (SFTSV), an emerging bunyavirus, causes severe fever with thrombocytopenia syndrome (SFTS), with a high fatality rate of 20%-30%. At present, however, the pathogenesis of SFTSV remains largely unclear and no specific therapeutics or vaccines against its infection are currently available. Therefore, animal models that can faithfully recapitulate human disease are important to help understand and treat SFTSV infection. Here, we infected seven Chinese rhesus macaques (Macaca mulatta) with SFTSV. Virological and immunological changes were monitored over 28 days post-infection. Results showed that mild symptoms appeared in the macaques, including slight fever, thrombocytopenia, leukocytopenia, increased aspartate aminotransferase (AST) and creatine kinase (CK) in the blood. Viral replication was persistently detectable in lymphoid tissues and bone marrow even after viremia disappeared. Immunocyte detection showed that the number of T cells (mainly CD8⁺ T cells), B cells, natural killer (NK) cells, and monocytes decreased during infection. In detail, effector memory CD8⁺ T cells declined but showed increased activation, while both the number and activation of effector memory CD4⁺ T cells increased significantly. Furthermore, activated memory B cells decreased, while CD80⁺/CD86⁺ B cells and resting memory B cells (CD27⁺CD21⁺) increased significantly. Intermediate monocytes (CD14⁺CD16⁺) increased, while myeloid dendritic cells (mDCs) rather than plasmacytoid dendritic cells (pDCs) markedly declined during early infection. Cytokines, including interleukin-6 (IL-6), interferon-inducible protein-10 (IP-10), and macrophage inflammatory protein 1 (MCP-1), were substantially elevated in blood and were correlated with activated CD4⁺ T cells, B cells, CD16⁺CD56⁺ NK cells, CD14⁺CD16⁺ monocytes during infection. Thus, this study demonstrates that Chinese rhesus macaques infected with SFTSV resemble mild clinical symptoms of human SFTS and provides detailed virological and immunological parameters in macaques for understanding the pathogenesis of SFTSV infection.

Effect of genomic variations in severe fever with thrombocytopenia syndrome virus on the disease lethality

Severe fever with thrombocytopenia syndrome virus (SFTSV), an emerging tick-borne bunyavirus, causes mild-to-moderate infection to critical illness or even death in human patients. The effect of virus variations on virulence and related clinical significance is unclear. We prospectively recruited SFTSV-infected patients in a hotspot region of SFTS endemic in China from 2011 to 2020, sequenced whole genome of SFTSV, and assessed the association of virus genomic variants with clinical data, viremia, and inflammatory response. We identified seven viral clades (I-VII) based on phylogenetic characterization of 805 SFTSV genome sequences. A significantly increased case fatality rate (32.9%) was revealed in one unique clade (IV) that possesses a specific co-mutation pattern, compared to other three common clades (I, 16.7%; II, 13.8%; and III, 11.8%). The phenotype-genotype association (hazard ratios ranged 1.327-2.916) was confirmed by multivariate regression adjusting age, sex, and hospitalization delay. We revealed a pronounced inflammation response featured by more production of CXCL9, IL-10, IL-6, IP-10, M-CSF, and IL-1β, in clade IV, which was also related to severe complications. We observed enhanced cytokine expression from clade IV inoculated PBMCs and infected mice. Moreover, the neutralization activity of convalescent serum from patients infected with one specified clade was remarkably reduced to other viral clades. Together, our findings revealed a significant association between one specific viral clade and SFTS fatality, highlighting the need for molecular surveillance for highly lethal strains in endemic regions and unravelled the importance of evaluating cross-clade effect in development of vaccines and therapeutics.

Immune escape mechanisms of severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome (SFTS), which is caused by SFTS virus (SFTSV), poses a serious threat to global public health, with high fatalities and an increasing prevalence. As effective therapies and prevention strategies are limited, there is an urgent need to elucidate the pathogenesis of SFTS. SFTSV has evolved several mechanisms to escape from host immunity. In this review, we summarize the mechanisms through which SFTSV escapes host immune responses, including the inhibition of innate immunity and evasion of adaptive immunity. Understanding the pathogenesis of SFTS will aid in the development of new strategies for the treatment of this disease.

Overview of the immunological mechanism underlying severe fever with thrombocytopenia syndrome (Review)

Severe fever with thrombocytopenia syndrome (SFTS) has been acknowledged as an emerging infectious disease that is caused by the SFTS virus (SFTSV). The main clinical features of SFTS on presentation include fever, thrombocytopenia, leukocytopenia and gastrointestinal symptoms. The mortality rate is estimated to range between 5-30% in East Asia. However, SFTSV infection is increasing on an annual basis globally and is becoming a public health problem. The transmission cycle of SFTSV remains poorly understood, which is compounded by the pathogenesis of SFTS not being fully elucidated. Since the mechanism underlying the host immune response towards SFTSV is also unclear, there are no effective vaccines or specific therapeutic agents against SFTS, with supportive care being the only realistic option. Therefore, it is now crucial to understand all aspects of the host-virus interaction following SFTSV infection, including the antiviral states and viral evasion mechanisms. In the present review, recent research progress into the possible host immune responses against SFTSV was summarized, which may be useful in designing novel therapeutics against SFTS.

Animal Model of Severe Fever With Thrombocytopenia Syndrome Virus Infection

Severe fever with thrombocytopenia syndrome (SFTS), an emerging life-threatening infectious disease caused by SFTS bunyavirus (SFTSV; genus Bandavirus, family Phenuiviridae, order Bunyavirales), has been a significant medical problem. Currently, there are no licensed vaccines or specific therapeutic agents available and the viral pathogenesis remains largely unclear. Developing appropriate animal models capable of recapitulating SFTSV infection in humans is crucial for both the study of the viral pathogenic processes and the development of treatment and prevention strategies. Here, we review the current progress in animal models for SFTSV infection by summarizing susceptibility of various potential animal models to SFTSV challenge and the clinical manifestations and histopathological changes in these models. Together with exemplification of studies on SFTSV molecular mechanisms, vaccine candidates, and antiviral drugs, in which animal infection models are utilized, the strengths and limitations of the existing SFTSV animal models and some important directions for future research are also discussed. Further exploration and optimization of SFTSV animal models and the corresponding experimental methods will be undoubtedly valuable for elucidating the viral infection and pathogenesis and evaluating vaccines and antiviral therapies.

Single-cell landscape of peripheral immune responses to fatal SFTS

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease with high fatality. Poor prognosis of SFTS has been associated with dysregulated host immunity; however, the immune patterns associated with pathophysiology involving SFTS exacerbation remain unclear. Here, we show that the single-cell landscape of peripheral immune responses is reprogrammed in SFTS and characterized by monocyte shift to an intermediate type along with complement activation, perturbation of plasmablast composition, and highly exhausted T cells, all correlated with lethal consequences. We identify the overexpression of interferon (IFN)-stimulated genes across most immune cell types after SFTSV infection, which are simultaneously related to older age, high viremia, and a hyperinflammatory response. A retrospective clinical study reveals no efficiency of IFN-α in treating SFTS. These data collectively support the intermediate monocytes and IFN-I-inducible plasmablasts to be major targets for SFTS virus infection, and they indicate the pivotal role of the IFN-I response in exacerbating hyperinflammation and lethal SFTS.

SAFA initiates innate immunity against cytoplasmic RNA virus SFTSV infection

Nuclear scaffold attachment factor A (SAFA) is a novel RNA sensor involved in sensing viral RNA in the nucleus and mediating antiviral immunity. Severe fever with thrombocytopenia syndrome virus (SFTSV) is a bunyavirus that causes SFTS with a high fatality rate of up to 30%. It remains elusive whether and how cytoplasmic SFTSV can be sensed by the RNA sensor SAFA. Here, we demonstrated that SAFA was able to detect SFTSV infection and mediate antiviral interferon and inflammatory responses. Transcription and expression levels of SAFA were strikingly upregulated under SFTSV infection. SAFA was retained in the cytoplasm by interaction with SFTSV nucleocapsid protein (NP). Importantly, SFTSV genomic RNA was recognized by cytoplasmic SAFA, which recruited and promoted activation of the STING-TBK1 signaling axis against SFTSV infection. Of note, the nuclear localization signal (NLS) domain of SAFA was important for interaction with SFTSV NP and recognition of SFTSV RNA in the cytoplasm. In conclusion, our study reveals a novel antiviral mechanism in which SAFA functions as a novel cytoplasmic RNA sensor that directly recognizes RNA virus SFTSV and mediates an antiviral response.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an RNA virus with a high fatality rate of up to 30%, which replicates exclusively in the cytoplasm. To date, many cytoplasmic RNA sensors were known to recognize SFTSV infection and trigger antiviral immune responses. Nuclear scaffold attachment factor A (SAFA) is a novel nuclear RNA sensor which can sense viral RNA in the nucleus and promote activation of antiviral immunity. However, there are no studies to investigate whether SAFA could detect cytoplasmic RNA virus infection. Here, we reported that SAFA was able to detect RNA virus SFTSV invasion. Under SFTSV infection, SAFA was retained in the cytoplasm and recognized SFTSV infection by interaction with SFTSV nucleocapsid protein (NP) and cytoplasmic SFTSV RNA directly. Importantly, SAFA recruited and promoted the activation of the STING-TBK1 signaling pathway-mediated antiviral immunity to suppress SFTSV infection. This study provides a further acquaintance in SAFA-mediated antiviral immune responses, illustrating the novel role of SAFA in sensing cytoplasmic SFTSV and mediates an antiviral response.

Unraveling the Underlying Interaction Mechanism Between Dabie bandavirus and Innate Immune Response

The genus Bandavirus consists of seven tick-borne bunyaviruses, among which four are known to infect humans. Dabie bandavirus, severe fever with thrombocytopenia syndrome virus (SFTSV), poses serious threats to public health worldwide. SFTSV is a tick-borne virus mainly reported in China, South Korea, and Japan with a mortality rate of up to 30%. To date, most immunology-related studies focused on the antagonistic role of SFTSV non-structural protein (NSs) in sequestering RIG-I-like-receptors (RLRs)-mediated type I interferon (IFN) induction and type I IFN mediated signaling pathway. It is still elusive whether the interaction of SFTSV and other conserved innate immune responses exists. As of now, no specific vaccines or therapeutics are approved for SFTSV prevention or treatments respectively, in part due to a lack of comprehensive understanding of the molecular interactions occurring between SFTSV and hosts. Hence, it is necessary to fully understand the host-virus interactions including antiviral responses and viral evasion mechanisms. In this review, we highlight the recent progress in understanding the pathogenesis of SFTS and speculate underlying novel mechanisms in response to SFTSV infection.

Host restriction of emerging high-pathogenic bunyaviruses via MOV10 by targeting viral nucleoprotein and blocking ribonucleoprotein assembly

Bunyavirus ribonucleoprotein (RNP) that is assembled by polymerized nucleoproteins (N) coating a viral RNA and associating with a viral polymerase can be both the RNA synthesis machinery and the structural core of virions. Bunyaviral N and RNP thus could be assailable targets for host antiviral defense; however, it remains unclear which and how host factors target N/RNP to restrict bunyaviral infection. By mass spectrometry and protein-interaction analyses, we here show that host protein MOV10 targets the N proteins encoded by a group of emerging high-pathogenic representatives of bunyaviruses including severe fever with thrombocytopenia syndrome virus (SFTSV), one of the most dangerous pathogens listed by World Health Organization, in RNA-independent manner. MOV10 that was further shown to be induced specifically by SFTSV and related bunyaviruses in turn inhibits the bunyaviral replication in infected cells in series of loss/gain-of-function assays. Moreover, animal infection experiments with MOV10 knockdown corroborated the role of MOV10 in restricting SFTSV infection and pathogenicity in vivo. Minigenome assays and additional functional and mechanistic investigations demonstrate that the anti-bunyavirus activity of MOV10 is likely achieved by direct impact on viral RNP machinery but independent of its helicase activity and the cellular interferon pathway. Indeed, by its N-terminus, MOV10 binds to a protruding N-arm domain of N consisting of only 34 amino acids but proving important for N function and blocks N polymerization, N-RNA binding, and N-polymerase interaction, disabling RNP assembly. This study not only advances the understanding of bunyaviral replication and host restriction mechanisms but also presents novel paradigms for both direct antiviral action of MOV10 and host targeting of viral RNP machinery.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging high-pathogenic bunyavirus listed by the World Health Organization as a top priority pathogen for research and development. Although SFTSV and related bunyaviruses emerging globally have raised serious public health concerns, specific antivirals or vaccines are currently unavailable and little is known on the virus-host interactions and viral replication mechanism. The nucleoprotein (N) is essential for bunyavirus replication by driving assembly of ribonucleoprotein (RNP), the RNA synthesis machinery and structural core of virions. Here we show that N proteins of SFTSV and related bunyaviruses can be targeted by host factor MOV10 in RNA-independent manner. Further, MOV10 can be induced specifically by the bunyaviruses and in turn restrict the viral replication and pathogenicity in vitro and in vivo. The anti-bunyavirus activity of MOV10 is independent of its helicase region and cellular interferon pathway. Instead, by its N-terminus, MOV10 binds to a protruding N-arm domain of N and blocks N polymerization, N-RNA binding, and N-polymerase interaction, disabling RNP assembly. This study provides a delicate model for host targeting of viral RNP machinery and sheds light on bunyaviral replication and host restriction mechanisms, which may promote specific antiviral therapy development.

The NF-κB inhibitor, SC75741, is a novel antiviral against emerging tick-borne bandaviruses

Severe fever with thrombocytopenia syndrome virus (SFTSV) and Heartland virus (HRTV) cause viral hemorrhagic fever-like illnesses in humans due to an aberrant host inflammatory response, which contributes to pathogenesis. Here, we established two separate minigenome (MG) systems based on the M-segment of SFTSV and HRTV. Following characterization of both systems for SFTSV and HRTV, we used them as a platform to screen potential compounds that inhibit viral RNA synthesis. We demonstrated that the NF-κB inhibitor, SC75741, reduces viral RNA synthesis of SFTSV and HRTV using our MG platform and validated these results using infectious SFTSV and HRTV. These results may lead to the use of MG systems as potential screening systems for the identification of antiviral compounds and yield novel insights into host-factors that could play role in bandavirus transcription and replication.

Severe Fever with Thrombocytopenia Syndrome Virus NSs Interacts with TRIM21 To Activate the p62-Keap1-Nrf2 Pathway

Nuclear factor erythroid 2-related factor 2 (Nrf2) dissociates from its inhibitor, Keap1, upon stress signals and subsequently induces an antioxidant response that critically controls the viral life cycle and pathogenesis. Besides intracellular Fc receptor function, tripartite motif 21 (TRIM21) E3 ligase plays an essential role in the p62-Keap1-Nrf2 axis pathway for redox homeostasis. Specifically, TRIM21-mediated p62 ubiquitination abrogates p62 oligomerization and sequestration activity and negatively regulates the Keap1-Nrf2-mediated antioxidant response. A number of viruses target the Nrf2-mediated antioxidant response to generate an optimal environment for their life cycle. Here we report that a nonstructural protein (NSs) of severe fever with thrombocytopenia syndrome virus (SFTSV) interacts with and inhibits TRIM21 to activate the Nrf2 antioxidant signal pathway. Mass spectrometry identified TRIM21 to be a binding protein for NSs. NSs bound to the carboxyl-terminal SPRY subdomain of TRIM21, enhancing p62 stability and oligomerization. This facilitated p62-mediated Keap1 sequestration and ultimately increased Nrf2-mediated transcriptional activation of antioxidant genes, including those for heme oxygenase 1, NAD(P)H quinone oxidoreductase 1, and CD36. Mutational analysis found that the NSs-A46 mutant, which no longer interacted with TRIM21, was unable to increase Nrf2-mediated transcriptional activation. Functionally, the NS wild type (WT), but not the NSs-A46 mutant, increased the surface expression of the CD36 scavenger receptor, resulting in an increase in phagocytosis and lipid uptake. A combination of reverse genetics and assays with Ifnar ^-/- mouse models revealed that while the SFTSV-A46 mutant replicated similarly to wild-type SFTSV (SFTSV-WT), it showed weaker pathogenic activity than SFTSV-WT. These data suggest that the activation of the p62-Keap1-Nrf2 antioxidant response induced by the NSs-TRIM21 interaction contributes to the development of an optimal environment for the SFTSV life cycle and efficient pathogenesis.IMPORTANCE Tick-borne diseases have become a growing threat to public health. SFTSV, listed by the World Health Organization as a prioritized pathogen, is an emerging phlebovirus, and fatality rates among those infected with this virus are high. Infected Haemaphysalis longicornis ticks are the major source of human SFTSV infection. In particular, the recent spread of this tick to over 12 states in the United States has increased the potential for outbreaks of this disease beyond Far East Asia. Due to the lack of therapies and vaccines against SFTSV infection, there is a pressing need to understand SFTSV pathogenesis. As the Nrf2-mediated antioxidant response affects viral life cycles, a number of viruses deregulate Nrf2 pathways. Here we demonstrate that the SFTSV NSs inhibits the TRIM21 function to upregulate the p62-Keap1-Nrf2 antioxidant pathway for efficient viral pathogenesis. This study not only demonstrates the critical role of SFTSV NSs in viral pathogenesis but also suggests potential future therapeutic approaches to treat SFTSV-infected patients.

Identifying target cells for a tick-borne virus that causes fatal hemorrhagic fever

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging disease in China, South Korea, and Japan caused by the tick-borne SFTS virus (SFTSV). Severe and fatal SFTS presents as a hemorrhagic fever characterized by high viral load, uncontrolled inflammatory response, dysregulated adaptive immunity, coagulation abnormalities, hemorrhage, and multiorgan failure with up to 33% case fatality rates (CFRs). Despite its public health significance in Asia, vaccines and specific therapeutics against SFTS are still unavailable. A better understanding of the pathogenesis of SFTS is crucial to improving medical countermeasures against this devastating disease. In this issue of the JCI, Suzuki and colleagues analyzed histopathological samples from 22 individuals who succumbed to SFTS, and identified antibody-producing B cell-lineage plasmablasts and macrophages as principal target cells for SFTSV infection in fatal SFTS. Their results suggest that SFTSV-infected post-germinal center B cells, plasmablasts, and macrophages affect systemic immunopathology and dysregulation, which likely leads to fatal outcomes.

Revisiting new tick-associated viruses: what comes next?

Tick-borne viral infections continue to cause diseases with considerable impact on humans, livestock, companion animals and wildlife. Many lack specific therapeutics and vaccines are available for only a few. Tick-borne viruses will continue to emerge, facilitated by anthroponotic factors related to the modern lifestyle. We persistently identify and are obliged to cope with new examples of emerging tick-borne viral diseases and novel viruses today. Many new strains have been detected in vertebrates and arthropods, some causing severe diseases likely to challenge public and veterinary health. This manuscript aims to provide a narrative overview of recently-described tick-associated viruses, with perspectives on changing paradigms in identification, screening and control.

Rescue of infectious Arumowot virus from cloned cDNA: Posttranslational degradation of Arumowot virus NSs protein in human cells

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to Africa and the Middle East, affecting both humans and ruminants. There are no licensed vaccines or antivirals available for humans, whereas research using RVF virus (RVFV) is strictly regulated in many countries with safety concerns. Nonpathogenic Arumowot virus (AMTV), a mosquito-borne phlebovirus in Africa, is likely useful for the screening of broad-acting antiviral candidates for phleboviruses including RVFV, as well as a potential vaccine vector for RVF. In this study, we aimed to generate T7 RNA polymerase-driven reverse genetics system for AMTV. We hypothesized that recombinant AMTV (rAMTV) is viable, and AMTV NSs protein is dispensable for efficient replication of rAMTV in type-I interferon (IFN)-incompetent cells, whereas AMTV NSs proteins support robust viral replication in type-I IFN-competent cells. The study demonstrated the rescue of rAMTV and that lacking the NSs gene (rAMTVΔNSs), that expressing green fluorescent protein (GFP) (rAMTV-GFP) or that expressing Renilla luciferase (rAMTV-rLuc) from cloned cDNA. The rAMTV-rLuc and the RVFV rMP12-rLuc showed a similar susceptibility to favipiravir or ribavirin. Interestingly, neither of rAMTV nor rAMTVΔNSs replicated efficiently in human MRC-5 or A549 cells, regardless of the presence of NSs gene. Little accumulation of AMTV NSs protein occurred in those cells, which was restored via treatment with proteasomal inhibitor MG132. In murine MEF or Hepa1-6 cells, rAMTV, but not rAMTVΔNSs, replicated efficiently, with an inhibition of IFN-β gene upregulation. This study showed an establishment of the first reverse genetics for AMTV, a lack of stability of AMTV NSs proteins in human cells, and an IFN-β gene antagonist function of AMTV NSs proteins in murine cells. The AMTV can be a nonpathogenic surrogate model for studying phleboviruses including RVFV.